In cementing casing or liner, wiper plugs are used to isolate the delivered cement from existing well fluids and to drive any leftover cement out of the casing or liner and through the cement shoe, which is a one way valve at the lower end of the casing or liner string. Some systems get by with only a single wiper plug. In those systems the cement is delivered on top of existing well fluids with no barrier. After the cement is delivered, the one wiper plug is dropped to displace the cement from the casing or liner and into the surrounding annulus. After that the cement shoe at the bottom of the string along with the wiper plug are simply milled up and the well is continued deeper.
In two wiper plug systems of the past, one of the concerns was to only drop one wiper plug at a time. Earlier designs of multi-plug systems used a system of two shear pins. The lower pin supported the lower wiper plug from the wiper plug above it. The upper pin held the upper plug to the tool body and was designed to shear at a higher pressure than the lower shear pin. A pump down plug seated in the tool to allow pressure to break the lower shear pin while claiming to keep the upper wiper plug in pressure balance. What was supposed to happen is that the lower pin sheared and the lower wiper plug launched. Then another pump down plug was landed to allow a net pressure force to be applied to the remaining wiper plug so that the upper shear pin that was rated higher than the lower shear pin could release. The upper wiper plug then was launched. This design is illustrated in Application WO 94/27026. The problem with this design is that if the lower shear pin didn't release when needed, pressure would build to the point of breaking the higher set upper shear pin and both wiper plugs would launch together. In other words, there was nothing to assure the upper wiper plug could not be launched with the lower wiper plug.
In an effort to address this issue U.S. Pat. No. 6,206,094 was designed to use a hydraulic system with metering capability to advance the lower wiper plug while it was still retained to the tool for a given travel distance at which point the lower wiper plug could launch. A first pump down plug allowed pressure to be applied to move a piston that moved the wiper plug at a controlled rate until it extended far enough from the tool housing to be released. A second pump down plug then allowed another piston to move at a regulated rate to advance the second wiper plug beyond the housing far enough so that it too could be launched. While this tool provided greater assurance of launching only one wiper plug at a time, it was complicated and involved rupture discs and hydraulic flow through metering orifices. It presented some risk for smooth operation as intended.
Other known wiper plug launching systems were the LFC Four Plug System offered by Baker Oil Tools that worked similarly to Application WO 94/27026 but used collets which became unsupported or sheared to trigger a release in conjunction with shear pins to hold a sleeve from moving where a collet became unsupported for release. Another similar design is U.S. Pat. No. 5,553,667 (FIG. 9). Other designs in this area include U.S. Pat. Nos. 5,803,173; 6,712,152; 6,698,513; 6,575,238; 6,681,860; 6,672,384 and 7,055,611.
What is needed and provided by the present invention is a wiper plug dropping tool that retains the pump down plugs and ensures the orderly release of the wiper plugs. It features a sleeve that is moved to release the lower wiper plug whose movement makes it possible to actually land another pump down plug in a proper position so that the release mechanism for the upper wiper plug can be actuated. Without movement of the release sleeve for the lower wiper plug there is no release of the upper wiper plug. These and other features of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings with the understanding that the full scope of the invention is measured by the claims that appear below.